1. Field of Technology
The present invention relates to a cathode ray tube apparatus for use in a television receiver set, a television monitor, or a computer display device.
2. Description of the Prior Art
Some typical prior art cathode ray tubes to which the present invention pertains will be discussed with the aide of FIGS. 1 to 4 of the accompanying drawings.
FIG. 1 illustrates, in horizontal sectional view, an in-line electron gun assembly used in one prior art cathode ray tube and mounted in the neck region thereof. The illustrated beam-producing electron gun assembly comprises three cathodes 1 disposed in in-line fashion in a horizontal direction (hereinafter referred to as "X-axis") perpendicular to the longitudinal axis Y of the cathode ray tube, which cathodes 3 constitute, together with a common control electrode or grid 2 and an accelerating electrode or grid 3, a front stage electrode triad. The electron gun assembly also comprises an anode 5 and a focusing electrode 4 positioned between the electrode triad and the anode 5.
The prior art cathode ray tube utilizing the above described electron gun assembly operates in the following manner. In order to eliminate the necessity of using a dynamic convergence circuit, an electron beam deflecting system used in the prior art cathode ray tube makes use of a self-convergence deflection yoke capable of distorting a horizontal deflection magnetic field so as to represent a pincushion pattern and also distorting a vertical deflection magnetic field so as to represent a barrel-shaped pattern. Therefore, some of the spots of electron beams cast on the phosphor screen of the cathode ray tube, which are located adjacent a peripheral area of the phosphor screen, tend to represent a generally oval or elliptical shape as shown in FIG. 2. This distortion of the electron beam spots results in a considerable reduction in resolution of a picture being reproduced on the viewing screen of the cathode ray tube particularly at the peripheral area thereof. In order to substantially eliminate the considerable reduction in resolution of the picture being reproduced, it is usual for the prior art cathode ray tube to have a so-called dynamic focusing system wherein a focusing voltage Vf to be applied to the focusing electrode 4 is superimposed with a modulating voltage Em which varies so as to increase at the peripheral area of the phosphor screen in synchronism with a horizontal deflection frequency, thereby to compensate for distortion in shape of the electron beam spots. In other words, correction of the oval or elliptical shape of the electron beam spots to a circular shape presented in a central area of the viewing screen as shown in FIG. 2 is carried out.
It has, however, been found that, although the use of the dynamic focusing system referred to above improves the resolution of the picture being reproduced on the viewing screen of the cathode ray tube, particularly that of the peripheral area of the reproduced picture, the convergence of the principal electron lens formed between the anode 5 and the focusing electrode 4 tends to vary with modulation of the focusing voltage Vf, resulting in a misconvergence in which, of the three electron beams of different colors (for example, red, green and blue), two electron beams of green and blue colors traveling on respective side of the electron beam of red color diverge laterally outwardly therefrom.
On the other hand, in order to render the respective spots of the electron beams impinging upon the peripheral area of the viewing screen to be circular in shape, an electron gun assembly has been proposed wherein electrodes forming a quadrupole electrode structure are disposed inside an electron gun assembly so that the trajectories of the electron beams traveling towards the viewing screen can be corrected electrostatically.
By way of example, the Japanese Laid-open Pat. No. 53-9464published Jan. 27, 1978, discloses the electron gun assembly of the type referred to above and as shown. FIG. 3 illustrates in a partially exploded view, a quadrupole electrode structure used in this prior art electron gun assembly for forming a quadrupole lens. Referring to FIG. 3, the quadrupole electrode structure comprises three cylindrical electrodes 6, 7 and 8 arranged in side-by-side relationship in a horizontal direction X, each of said cylindrical electrodes 6 to 8 having vertically upwardly and downwardly oriented openings 6a and 6b, 7a and 7b, 8a 8b, defined therein. The electron gun assembly shown therein also comprises electrode strips 9 and 10 positioned and supported so as to traverse immediately above the openings 6a7a and 8a and below the openings 6b, 7b and 8b, respectively, so that electromagnetic field developed inside each of the openings 6a to 8a of the respective cylindrical electrodes 6 to 8 can form an electromagnetic quadrupole electrode assembly.
The electrode structure disclosed in the Japanese Laid-open Patent Publication No. 53-9464 has been found difficult to assembly into a unitary structure and has also been found to be complicated and time-consuming to fabricate. Moreover, since the electrode strips 9 and 10 positioned on respective sides of the cylindrical electrodes 6 to 8 in parallel relation to each other are utilized as respective electrodes common to all of the cylindrical electrodes 6 to 8, the misconvergence tends to occur depending on the voltage applied between the cylindrical electrodes 6 to 8, with the consequence that the convergence characteristic tends to be impaired.
The electron gun assembly utilizing the quadrupole electrode structure is also disclosed in the Japanese Laid-open Patent Publication No. 61-39347 published Feb. 26, l986. This electron gun assembly is shown in FIGS. 4(a) and 4(b) in schematic side view and in schematic front elevational view, respectively. This electron gun assembly includes a quadrupole electrode structure which is defined by a pair of vertical electrode pieces 11, 12 or 13 spaced apart from each other and positioned on respective side of the path of travel of the respective electron beam, and a pair of horizontal electrode pieces 14 common to all of the pairs of the vertical electrode pieces 11 to 13 and positioned immediately above and below the pairs of the horizontal electrode pieces 11 to 13.
The quadrupole electrode structure used in the electron gun assembly according to the Japanese Laid-open Patent Publication No. 61-39347 has a problem similar to that inherent in the quadrupole electrode structure shown in and described with reference to FIG. 3 since the pair of the horizontal electrode pieces 14 are utilized for all of the electron beams.